Eh... well.. That depends. Personally.... I'd get a second 680. The 770 is slightly better than the 680 (like 5~10% better), but, getting a second 680 is the best value. I don't think spending £1100 on two 780s plus blocks over SLI 680 is worth it.

Like I said... I'd get another 680, water blocks for both, and any money left, I would update something else my rig needed or just keep it, in your case, the watercooling loop.